1. Field of the Invention
The present invention relates to an objective lens, and a method of fabricating such an objective lens. Particularly, the present invention relates to an objective lens as the optical pickup lens for writing/reading information to/from an optical recording medium such as optical disks and magneto-optical disks, and a method of fabricating such an objective lens.
2. Description of the Background Art
As conventional information recording media, various optical recording media such as optical disks, magneto-optical disks and optical cards are proposed. The so-called optical pickup that directs light onto an optical recording medium is employed as the system of writing/reading information to/from such an optical recording medium.
In optical pickup, the light directed onto the optical recording medium is collected by an objective lens, and the focus is located on the plane of the optical recording medium. In this system, the diameter of the focused light beam can be reduced by increasing the numerical aperture (NA) of the objective lens to improve the signal recording density of the optical recording medium.
When the numerical aperture is to be increased with the objective lens formed of one lens, i.e., the so-called single lens, the refractive power must be increased. The rate of curvature of the objective lens becomes smaller as the refractive power is increased, so that the positioning accuracy of the refractive planes with respect to each other must be improved. Therefore, increase of the numerical aperture was limited to approximately 0.6 with the single lens.
In contrast to the single lens, the numerical aperture can be increased with the so-called two-lens, i.e. a group formed of two lenses. The two-lens will be described in the following.
FIG. 16 is a schematic diagram of an objective lens formed of the conventional two-lens. Referring to FIG. 16, the objective lens is formed of a two-lens 60. Two-lens 60 includes a first lens 61 and a second lens 62. First lens 61 includes a first plane 63 on which the laser beam emitted from a semiconductor laser is incident, and a second plane 64 facing second lens 62. Second lens 62 includes a third plane 65 on which the laser beam passing through first lens 61 is incident and a fourth plane 66 facing an optical recording medium.
In two-lens 60 of the above structure, the numerical aperture can be easily increased since the refractive power of the laser beam can be diffused.
In the objective lens of FIG. 16, relative inclination and offset in the lens axis occurs between first and second lenses 61 and 62 caused by error in the working accuracy of the lens holder that supports first and second lenses 61 and 62 or fixture error in fixing the lens. A method to adjust these errors is required.
Japanese Patent Laying-Open No. 7-220286 discloses a method of adjusting lens inclination in the so-called single lens. However, this method cannot accommodate adjustment of inclination between two lenses or offset in the lens axis between two lenses (decentration adjustment).
Japanese Patent Laying-Open No. 10-255304 discloses adjustment of inclination and decentration between two lenses employed as objective lens. However, this method induces the possibility of the focal point formed by the two lenses being altered due to change in the distance between the lenses during adjustment of inclination and decentration. In this case, spherical aberration occurs to degrade the performance of the objective lens.
The present invention is directed to solve the above problems, and an object is to provide an objective lens that allows inclination adjustment and decentration adjustment without altering the distance between two lenses, and that allows data to be written and read reliably, and a method of fabricating such an objective lens.
According to an aspect of the present invention, a fabrication method of an objective lens includes the steps of: (1) attaching a first lens to a first holder, (2) adjusting the optical axis of the second lens with respect to the optical axis of the first lens while sliding the second lens at the lens mount surface of the second holder, and attaching the second lens to the second holder so that the angle is substantially 0 degree, (3) forming contact between the first plane of the first holder perpendicular to the reference axis and the second plane of the second holder perpendicular to the reference axis, (4) performing decentration adjustment so that the optical axis of the first lens matches the optical axis of the second lens by sliding the second holder with respect to the first holder in a state where the first and second planes are in contact with each other, and (5) fixing the second holder subjected to decentration adjustment with respect to the first holder.
According to the fabrication method of an objective lens including the foregoing steps, decentration adjustment is performed with the first holder sliding with respect to the second holder. Since the first plane and the second plane functioning as sliding planes are perpendicular to the reference axis, the first and second lenses respectively move in a direction perpendicular to the reference axis. Therefore, the distance from the sliding plane of the first and second lenses is not altered during decentration adjustment. As a result, decentration adjustment can be carried out only by the aforementioned sliding as long as the angle between the optical axis of the first lens to the optical axis of the second lens is substantially 0 degree, i.e. in parallel.
Since the second lens is slid on the lens mounting surface of the second holder in attaching the second lens, the distance between the first and second lenses does not change in the attaching operation of the second lens. Therefore, the distance between the first and second lenses does not change during adjustment of inclination of the second lens.
As a result, the distance between the first and second lenses is always constant. Thus, an objective lens is provided that allows data to be read and written reliably.
The step of attaching the second lens to the second holder preferably includes the steps of directing a laser beam onto a reference plane provided in the first lens and observing the reflected laser beam to measure a first angle between the optical axis of the first lens and the reference axis, directing a laser beam onto a reference plane provided at the second lens and observing the reflected laser beam to measure a second angle between the optical axis of the second lens and the reference axis, and positioning the second lens by sliding the second lens on the lens mounting plane of the second holder so that difference between the first angle and the second angle is substantially 0 degree and attaching the second lens to the second holder.
By measuring the angle between the optical axis of the first lens and the reference axis and also the angle between the optical axis of the second lens and the reference axis using a laser beam, the angles can be measured in accuracy. The second lens can be positioned in further accuracy. As a result, an objective lens is provided that can read/write data reliably.
The method of performing decentration adjustment preferably includes the step of sliding the second holder to position the second holder so that the shape of the beam spots of the laser beams passing through the first lens and the second lens are point-symmetric.
Preferably, the objective lens is an objective lens for optical pickup.
According to another aspect of the present invention, an objective lens includes a first holder, a first lens, a second holder, and a second lens. The first holder has a first plane perpendicular to the reference axis. The first lens has an optical axis, and is attached to the first holder. The second holder has a second plane perpendicular to the reference axis, and is fixed with respect to the first holder in a state where the first and second lenses are in contact. The second lens has an optical axis identical in axis to the optical axis of the first lens, and is attached to the second holder.
According to the objective lens of the above structure, the first and second lenses are attached to the first and second holders, respectively, so that the distance between the first lens and the second lens in adjusting inclination will not be altered by adjusting the inclination of the second lens on the second holder.
Furthermore, since the first and second planes perpendicular to the reference axis are in contact, the distance between the first lens and the second lens during decentration adjustment will not change if the decentration adjustment is performed by sliding the second plane with respect to the first plane. The distance between the first lens and the second lens is constant, so that the position of the focal point will not change. As a result, an objective lens can be provided that allows data to be read/written reliably.
Preferably, the second holder includes a spherical concavity. The second lens has a spherical convexity having a radius of curvature identical to the radius of curvature of the spherical concavity. The second lens is positioned so that the convexity of the second lens fits into the concavity of the second holder.
Since the radius of curvature of the concavity of the second holder is identical to the radius of curvature of the convexity of the second lens, they fit together with no gaps. Therefore, the second lens is reliably positioned with respect to the second holder.
Preferably, at least one of the first and second lenses has a plane facing an optical recording medium.
Preferably, the objective lens is an objective lens for optical pickup.
According to a further aspect of the present invention, a fabrication method of an objective lens includes the steps of: (1) attaching a first lens to a first holder having a first plane so that the optical axis of the first lens is substantially perpendicular to the first plane, (2) attaching a second lens to a second holder having a second plane so that the optical axis of the second lens is substantially perpendicular to the second plane, (3) forming contact between the first and second planes, (4) rotating the second holder with respect to the first holder to adjust the angle of the optical axis of the second lens with respect to the optical axis of the first lens in the state where the first and second planes are in contact and minimizing the angle, and (5) performing decentration adjustment by sliding the second holder with respect to the first holder so that the optical axis of the first lens matches the optical axis of the second lens in the state where the first and second planes are in contact.
Preferably, the step of performing decentration adjustment includes the step of performing decentration adjustment so that the coma of the two-lens formed of first and second lenses is minimized.
Preferably, the step of attaching the first lens to the first holder includes the step of attaching the first lens to the first holder to form a first lens unit, and measuring the angle of the first plane to the reference plane of the first lens. The step of attaching the second lens to the second holder includes the step of attaching the second lens to the second holder to form a second lens unit, and measuring the angle of the second plane to the reference plane of the second lens. The step of forming contact between the first and second lenses includes the steps of selecting one first lens unit whose angle between the first plane and the reference plane of the first lens corresponds to a constant value out of a plurality of first lens units fabricated by the step of attaching the first lens, selecting one second lens unit whose angle between the second plane and the reference plane of the second lens corresponds to a predetermined value out of a plurality of second lens units fabricated by the step of attaching the second lens, and forming contact between the first and second planes of the selected first and second lens units.
Preferably, the objective lens is an objective lens for optical pickup.
According to still another aspect of the present invention, an objective lens includes a first lens, a second lens, a first holder, and a second holder. The first holder holds the first lens. The second holder holds the second lens. The first and second holders have first and second planes substantially perpendicular to the optical axis. The first plane is in contact with the second plane.
Preferably, at least one of the first and second lenses has a surface facing an optical recording medium.
Preferably, the objective lens is an objective lens for optical pickup.
According to a still further aspect of the present invention, a fabrication method of an objective lens includes the steps of (1) attaching a first lens to a first holder having a first plane, (2) attaching a second lens to a second holder having a second plane, (3) forming contact between the first plane and the second plane, (4) performing decentration adjustment so that the optical axis of the first lens matches the optical axis of the second lens by sliding the second holder with respect to the first holder in the state where the first plane and the second plane are in contact.
According to yet a further aspect of the present invention, a fabrication method of an objective lens includes the steps of (1) attaching a first lens to the first holder having a first plane so that the optical axis is substantially perpendicular to the first plane, (2) attaching the second lens to a second holder having a second plane so that the optical axis is substantially perpendicular to the second plane, (3) forming contact between the first plane and the second plane, and (4) rotating the second holder with respect to the first holder to adjust the angle of the optical axis of the second lens with respect to the optical axis of the first lens in the state where the first and second planes are in contact and minimizing the angle.